The present invention relates to a process for preparing fluoral hydrate or hexafluoracetone hydrate, and more particularly, it relates to processes for the preparation of these hydrates in pure form from hemiacetals.
The fluoral and hexafluoroacetone hydrates are useful, particularly as intermediates for the synthesis of 2,2,2-trifluoroethanol and of 1,1,1,3,3,3-hexafluoroisopropanol, which are themselves useful starting materials for the synthesis of anaesthetics.
Fluoral hydrate is used herein to mean all of the following compounds described in the literature: ##STR2##
Hexafluoracetone hydrate has the structure of a gem-diol: ##STR3## See W. J. Middleton and D. W. Wiley, 86 J. A. C. S. (1964) 4948, and C. G. Krespan and W. J. Middleton, Fluorine Chemistry Reviews 1(1), 145-196 (1967).
These hydrates containing trifluoromethyl radicals, as well as the hydrates containing chlorodifluoromethyl or dichlorofluoromethyl radicals which are present in the crude products, have melting points of the order of 50.degree. to 60.degree. C. and sublime at their melting points, which makes their separation by distillation impossible. See French Patent Application No. 2,493,836.
French Patent Application No. 2,493,836 describes a technique for purifying hexafluoroacetone containing chlorofluoroacetones. In essence, this consists of a chemical degradation, with the aid of basic compounds, such as calcium hydroxide, of the chlorinated by-products to produce compounds which can be separated by decantation after addition of an inorganic material, such as calcium chloride, which causes demixing of the liquid mixture. A pure hexafluoroacetone hydrate is then obtained. This process, which is not applicable to fluoral, requires a suitable treatment of the effluent water. In addition, this method is difficult to operate continuously. It is furthermore known from J. P. Guthrie, 53 Can. J. Chem. (1975) 898-906, that it is necessary to use a large quantity of water (a water/hemiacetal molar ratio greater than 50) to obtain an aldehyde hydrate from the corresponding hemiacetal.